The present invention relates to optical prisms, which constitute an element of an optical system for projecting image provided by an image display device, such as an LCD, onto the retina of the eye. Such an optical prism is a main optical element of an image display unit, which is supported as a head mounted display (abbreviated HMD) on a frame member at a predetermined position thereof, the frame member being mounted on the head of the observer for singe- or double-eye observation of image.
Up to date, development of such HMDs is in progress. Devices of this type are fining increasing applications to various fields, such as medical field, construction field, educational field and game or amusement field.
Another recent trend is to develop devices which can display high quality images compatible with VGA and those which serve as displays of OA (office automation) units or the like dealing with information.
Many HMDs currently proposed or being developed are of double-eye type, comprising a left and a right image display units corresponding to and observed by the left and right eyes, respectively. For OA and like purposes, single-eye type HMDs are also proposed, which permits observation of image display with one eye while permitting forward and near viewing fields to be secured with the other eye for operating a keyboard or like operating part at the same time.
In OA and other fields, there is a demand for HMDs which are compact and convenient to use and also those which can be worn just like glasses.
However, general household HMDs are still in the stage of development, and no HMD which is not only convenient as complete product but can give sufficient solutions to various specific technical problems in manufacture, has been developed.
Japanese Patent Laid-Open No. 8-234137 discloses an optical prism of reflecting light incident on its light incidence surface suited for incidence of light from the display face of a display element such as an LCD at least twice within itself and emitting the reflected light as emitted light to the outside for being led to an observer's eye pupil.
This disclosed optical system has an integral structure having a plurality of non-spherical reflecting surfaces, and is thought to meet various optical requirements when constructed as an observation system.
Japanese Patent Laid/Open No. 9-73005 discloses an optical prism of like type, which has positioning projections formed on its sides or its securing to a different part.
However, when constructing an HMD by employing any of the above optical prisms, the optical prism and the liquid crystal display element should be highly accurately held in a regular position relation to each other in order to ensure accurate projection of image from the liquid crystal display element onto the viewer's retinas. It is a current technical subject to facilitate manufacture of optical prism while ensuring the above positioning accuracy.
In the meantime, recently optical prisms are usually manufactured by the injection molding process. The injection molding process permits obtaining molding products having a very great variety of shapes by using slide mechanisms.
FIGS. 15(a) to 15(c) schematically show a die having the above slide function. Specifically, FIG. 15(a) shows the die in a state that slide parts S are projecting into a cavity C of the die so that the cavity C has a shape corresponding to the shape of the molding product. FIG. 15(b) shows the die in a state that a molding product of a plastic or like material is being taken out from the cavity C with the slide parts S retreated therefrom. FIG. 15(c) shows the positional relation of the slide parts S and the cavity C of the die shown in FIG. 15(a) but viewed from a different position.
Prisms having various shapes can be molded by using dies having the above slide function. Such dies are well known in the art, but can be used to mold the prism according to the present invention.
FIGS. 16(a) to 16(c) schematically show a mechanism for moving slide parts of a die having a slide mechanism. This mechanism has inclined angular pins AP. The angular pins AP are adapted such that their relative displacement from their state in thorough holes formed in slide parts SL to a state out of the thorough holes, causes a retreat of the slide parts SL from a die cavity C.
In the state shown in FIG. 16(a), a fixed die FD and a movable die MD are in the close proximity of each other. In this state, the angular pins AP are deeply inserted in the thorough holes of the slide parts SL. The slide parts SL are thus projected into the cavity C, thus defining a cavity shape complementary to a complicated molding product shape.
In the state shown in FIG. 16(b), the fixed and movable dies FD and MD have been relatively displaced away from each other. As a result of this relative displacement, the inclined angular pins AP have gotten out of the die cavity C. The slide parts SL thus have retreated along and eventually got out of the die cavity C.
In the state shown in FIG. 16(c), ejector pins EP have been projected from the state shown in FIG. 16(b), thus taking out a molded product MM having undercuts UC formed by the slide parts SL.
By using the injection molding process adopting the side mechanism as described above, it is possible to relatively readily and efficiently manufacture optical prisms having various shapes.